This invention relates to thermal processes which utilize gas fired burners to generate heat. The invention does not include systems which use solid, i.e. coal, or liquid fuels to generate heat although certain principles set forth below are applicable to solid and liquid fuel fired systems. The reason for this distinction is that in liquid and solid fuels, nitrogen can be chemically bonded to carbon and hydrogen within the fuel. Because of this chemical bond, nitrogen within the fuel is in a reactive state which can more easily form nitrous oxides. (As used herein, NO.sub.x means the various forms of nitrous oxides such as NO, NO.sub.2, N.sub.2 O, etc.) In a gas fired system such as a system utilizing natural gas, nitrogen is not generally present in the fuel. Instead, all the nitrous oxides are formed from the nitrogen within the combustion air which is generally in its unreacted molecular state. As used herein, "gas fired system" means a fuel fired combustion system using natural gas (including methane and small percentages of other elements commonly referred to as "street gas") and its higher order hydrocarbon derivatives such as butane, propane, etc. This invention relates to gas fired systems.
A tremendous effort has been expended in an attempt to reduce NO.sub.x emissions in gas fired systems. The art of reducing NO.sub.x emissions has advanced to the point where NO.sub.x in gas fired systems can be reduced to as low as 20 ppm (parts per million). As a basis for comparison, new, proposed regulations in certain regions of the United States such as California, are being contemplated which would limit the emissions for industrial processes to 9 ppm, a level which, before the present invention, was not obtainable.
The activity in the field of NO.sub.x reduction for industrial systems has been so extensive that it is not practical to cite in this Background section specific articles or specific prior art patents. For purposes of explaining the present invention and distinguishing it from the prior art, the various approaches heretofore used for reducing NO.sub.x emissions in industrial processes can be categorized and defined as follows:
A) There have been numerous attempts made to modify burners to reduce localized flame temperatures. These efforts includes use of excess air in the burners, staging the combustion instituted at the burner to occur in steps, modifying the air/gas mixing pattern, etc.
B) A second type of approach has been to adopt modifications in the combustion system to suppress the temperature of the products of combustion after they normally occur. Such types of modifications include use of water or steam injection into the flame, flue gas recirculation or recycling and process heat transfer related changes.
C) The third fundamental approach may be defined as post combustion flue gas treatment and would include process such as catalytic reduction of NO.sub.x in the presence of reducting gases such as ammonia, hydrogen and carbon monoxide, etc. This is the so-called reburn approach which basically accepts the fact that NO.sub.x formation will inherently occur in the combustion process and then treats the nitrous oxides like any other effluent which is to be cleansed. However, reburning creates its own problems which have to be solved properly to make sure that what is produced in the reburn is not worse than that which otherwise existed.
Because some characteristics of the present invention could conceivably be asserted to bear some resemblance to categories A or B, some further comment may be in order. Basically, given a gas composition that contains nitrogen and oxygen, it is inevitable that nitrous oxides will form if the gases are in the presence of one another for extended time periods at certain elevated temperatures, i.e. above 2800.degree. F., for a reaction time as short as a few hundred milliseconds. That is, composition, temperature and reaction time are the three variables which produce NO.sub.x. Now, it is difficult to maintain the reaction zone temperature and residence time at low enough values at all times during the combustion and post combustion steps. For example, when the combustion zone temperature reductions are attempted by one of the techniques mentioned in subparagraph B above, it is very difficult to reduce the high temperature reaction zones at the residence times necessary to achieve low enough levels of NO.sub.x to result in significantly reduced NO.sub.x emissions. Thus, when staged combustion processes such as discussed in subparagraph A above are used, it may be possible to reduce the NO.sub.x levels in the first step of the staged combustion. However, afterburning of the products of combustion within the same general combustion zone will then still result in formation of unacceptable levels of NO.sub.x. In summary, all of the prior art processes discussed above are inherently defective in that the NO.sub.x is still being formed and the solution employed is to reduce the severity of the formation which, while "doable", cannot be done to produce the low levels of NO.sub.x which new regulations are going to specify. Further, in most industrial processes, it is now common practice to obtain high fuel efficiencies by preheating the combustion and/or even enriching the oxygen content of the combustion air supplied to the burner. Each of these practices increases flame temperature significantly and this in turn results in considerably higher NO.sub.x formation.
Apart from discussing any of the prior art NO.sub.x processers, there is published literature and prior art workings which can establish the following "facts":
I) At stoichiometric proportions of fuel and air, it is known that significant nitrous oxide emissions will not occur below reaction temperature of approximately 2800.degree. F.
II) It is known to operate burners at sub-stoichiometric air/fuel ratios and burners have been developed which will so operate at such ratios.
III) It is known that the actual flame temperature of burners operated at substoichiometric ratios of air to natural gas will produce lower peak temperatures than when the burners are operated at stoichiometric or excess air conditions.
IV) When a burner is operated at sub-stoichiometric conditions, a reducing atmosphere rich in reducing combustibles will be generated.
The above "facts" are known in the art but only the "fact" identified as I was specifically developed for nitrous oxide emissions. "Facts" II, III and IV are known and have been developed for the industrial furnace art.